In a building, an enclosure, created by building components such as a suspended ceiling or false floor, and used for the movement of environmental air. Note 1: A plenum may be used to contain communications and power cables, e.g., to reach equipment installed in open office or laboratory space. Note 2: Cables installed in plenums must meet applicable environmental and fire protection regulations. This may mean enclosing them in suitable ducts or using cables having jackets and other components made of materials that are resistant to open flame and are non-toxic at high temperatures.